I was having the same issue when trying to drag a widget containing a wysiwyg field onto a sidebar. Some testing proved that it was the presence of the Visual tab in the wysiwyg editor. If I changed it to show only the Text tab, the error went away. But I opted to downgrade ACF instead. 5.6.1 works for me, but 5.6.2 and 5.6.3 both generate the “Uncaught TypeError: Cannot read property ‘onpageload’ of undefined” error.
